| The trend of automotive weight reduction inspires the extension usage of magnesium alloy for the body-in-white manufacturing for the advantages of low density, high strength-to-weight ratio, high stiffness-to-weight ratio, high damping capacity, easy machining and recycle?But welding of magnesium alloy led to serious and uncertain electrode burning, which resulted in short electrode life and inconsistent weld quality. Studying the failure of electrode, investigating the effects of electrode burning on welding quality are not only useful for improving mechanical properties of the electrode,extending the electrode life, but also very important to promoting the automotive industry application of magnesium alloys.Firstly, the appropriate welding parameters was selected with nugget diameter, shear force, and splash situation as the evaluation content, it is concluded that the best process parameters for the welding current was 22000 A, welding time was 110 ms, electrode force of 3.5k N. To study the influence of two different surface conditions on electrode life welded by using optimum process parameter, The results show that the electrode life of RSW magnesium with the mechanically grinded surface was about 420 points, due to the influence of the oxide film, the electrode life with original surface was about 375 points.Secondly, to observe the failure process of electrode in spot welding magnesium alloy, electrode tip diameter was recorded by the method of carbon imprint.Morphologies of electrode face, microstructure and micro-hardness of electrode tip were investigated by OM, SEM, XRD, micro-hardness tester. The results show that the process of continuous spot welding, made the electrode diameter add to 11 mm and electrode pit, which greatly influenced the current pinch effect on electrode face. At the same time,with the increase of weld spots, the local welding occurred between electrode face and magnesium alloy made a large area peel off when the electrode was lifted, which enlarged the plastic deformation and wear heavier; Additionally, magnesium spread in the copper matrix and the depth was about 16 microns when electrode was failure. Finally, the Cu2 Mg was brought out in the outermost layers of the electrode tip.The change of microstructure shows that the electrode recrystallization was generated, the hardness of electrode tip was reduced. Because of the changes of electrode properties, the current density is decreased and effective pressure is not enough, the nugget size and shearing force can't meet the standard requirements, which means electrode invalidation.Finally, the effects of electrode burning on surface forming, nugget size and shear force in continuous spot welding of magnesium alloy was researched. The results show that with the increase of weld points, the spot indentation became larger,and the surface forming became deteriorated. The nugget size reduced and the morphology of the nugget became dual-core shape. More cracks, shrinkage cavities and shrinkage porosities of joints were found in the REW magnesium alloy, the plastic ring and the equiaxed grain area decreased, but the columnar grain area increased.The shear force decreased integrally. The failure mode of shear fracture transferred from button fracture into interfacial fracture, when the nugget diameter was about 4mm and shear force was about1200N. |
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